A fuel cell is an electrochemical cell which consumes fuel and an oxidant on a continuous basis to generate electrical energy. The fuel is consumed at an anode and the oxidant at a cathode. The anode and cathode are placed in electrochemical communication by an electrolyte. One typical fuel cell employs a phosphoric acid electrolyte. The phosphoric acid fuel cell uses air to provide oxygen as an oxidant to the cathode and uses a hydrogen rich stream to provide hydrogen as a fuel to the anode. After passing through the cell, the depleted air and fuel streams are vented from the system on a continuous basis.
A typical fuel cell power plant comprises one or more stacks of fuel cells, the cells within each stack being connected electrically in series to raise the voltage potential of the stack. A stack may be connected in parallel with other stacks to increase the current generating capability of the power plant. Depending upon the size of the power plant, a stack of fuel cells may comprise a half dozen cells or less, or as many as several hundred cells. Air and fuel are usually fed to the cells by one or more manifolds per stack.
In each of the fuel cells, waste heat is a by-product of the steam reforming process for conversion of fuel to a hydrogen rich steam, electrochemical reactions and the heat generation associated with current transport within the cell components. Accordingly, a cooling system must be provided for removing the waste heat from a stack of fuel cells so as to maintain the temperature of the cells at a uniform level which is consistent with the properties of the material used in the cells and the operating characteristics of the cells.
In the stack, where the chemical reactions take place, water is used to cool the stack and generate steam to be used in the furnace, where chemical reactions occur to generate hydrogen. The waste heat, which is at around 500° F. and includes water, exit air and depleted fuel, is directed to a waste heat recovery loop to provide the customer with low grade heat (i.e. 20-80 kw). The heat recovery loop often includes a condenser coupled with a glycol loop and a heat exchanger that couples the water system with the glycol loop. The customer can also get high grade heat (i.e. 284 kw) via the water which receives heat from the stack cooling loop. The heat exchangers that allow the customer to get the high and low grade heat are referred to as the customer water interface.